1. Field of the Invention
The present invention relates to a receiving apparatus in digital terrestrial broadcasting.
2. Description of the Related Art
A receiving apparatus disclosed in Japanese Unexamined Patent Publication No. 2001-77648 has been known as a conventional one. Hereinafter, prior art relating to the present invention will be described with reference to the drawings.
FIG. 1 is a configuration view of a receiving apparatus according to a first conventional example. The receiving apparatus includes an antenna 1, an RF amplifier 2, mixers 7, 8, a PLL 9, a divider 10, low pass filters (LPF) 11, 12, phase shifters 13, 14, an adder 15, a band pass filter (BPF) 16, a base band amplifier (BB amplifier) 17, an LPF 18, an AD converter (ADC) 19, an OFDM demodulator 20 and a transport stream output terminal (TS output terminal) 21.
An ISDB-T signal is inputted to the antenna 1. ISDB-T is a standard for digital terrestrial broadcasting which will start from 2003 in Japan. In a UHF band, TV broadcasting will start that has thirteen segments with a bandwidth of 429 kHz that are connected and subjected to OFDM modulation and a resultant OFDM signal is transmitted at a band of 6 MHz. A stationary receiver substituted by a conventional home TV can receive the whole thirteen segments and enjoy the high-vision broadcasting service. A transmission system in which only one segment serving as a central segment of thirteen segments can be received is also provided. In this case, services for a mobile receiver with a simple structure can be provided.
On the other hand, in the VHF band, sound broadcasting that has eight or twelve segments with a bandwidth of 429 kHz are connected and subjected to OFDM modulation and a resultant signal is transmitted at a band of 4 or 6 MHz. The sound broadcasting provides a service that each of the segments is dependent. In a specification of this case, any one segment is cut and partially received. Similar to TV, services for a mobile receiver with a simple structure can be provided.
The receiving apparatus with the structure shown in FIG. 1 is a receiving apparatus which performs partial reception of one segment in the above-described TV broadcasting and sound broadcasting. An ISDB-T signal received by the antenna 1 is amplified by the RF amplifier 2 and a resultant amplified signal is inputted to a quadrature mixer formed of the mixers 7 and 8. An oscillation signal with a predetermined frequency is generated at the PLL 9 and the oscillation signal is supplied to the divider 10. The divider 10 divides the signal into two signals that have frequencies higher than a center frequency of the segment to be partially received by 500 kHz and whose phases are different from each other by 90°. The divided signal is supplied to the mixers 7 and 8 as a local oscillation signal.
A method of dividing frequency into four is utilized for the divider 10 in order to obtain a phase difference of 90° with high precision by a frequency-dividing operation. When a segment with a center frequency of fRF is received, an oscillation frequency of the PLL 9 is 4×(fRF+500 kHz). A range of oscillation frequency of the PLL 9 can be made narrow by using frequency-halving. In accordance with the frequency-dividing method, a balance signal with a frequency which is twice as large as a required frequency is generated and a positive and negative phase signals are divided into two, so that signals with a phase difference of 90° are generated. In accordance with this frequency-dividing method, a quadrature precision is inferior as compared to the method of dividing frequency into four. In this case, an oscillation frequency of the PLL 9 is 2×(fRF+500 kHz). In this way, the mixer circuits 7 and 8 convert received signals into two intermediate frequency signals whose phases are different from each other by 90°. As a result, signals of an intermediate frequency of 500 kHz with the I axis and Q axis perpendicular to each other are generated.
Then, the intermediate frequency signals from the mixers 7 and 8 are supplied via the LPF 11 and LPF 12 to the phase shifters 13 and 14. The phase shifter 13 phase-shifts an I axis intermediate frequency signal serving as an output of the mixer 7 by φ. The phase shifter 14 phase-shifts a Q axis intermediate frequency signal serving as an output of the mixer 8 by (φ+90°). The phase-shifted intermediate frequency signals are supplied to the adder 15. The adder 15 adds an output of the phase shifter 13 to an output of the phase shifter 14 so as to output an intermediate frequency signal that an image signal component is cancelled and only a desired signal component is contained.
A quadrature mixer and an image rejection mixer that perform an important function in the receiving apparatus will be described. FIGS. 2 and 3 are functional block diagrams showing a structure of the image rejection mixer. FIG. 2 is a functional block diagram in a case of canceling a lower side band. FIG. 3 is a functional diagram in a case of canceling an upper side band.
An image rejection mixer 30A shown in FIG. 2 is configured so as to include a first mixer 31a, a second mixer 31b, a local oscillator 32, a first phase shifter 33, a first low pass filter (LPF) 34a, a second LPF 34b, a second phase shifter 35A and an adder 36. When a signal with an angular frequency of ω is subjected to amplitude modulation by using an input signal with an angular frequency of p, a cos(ω−p) t component, a cos ωt component and a cos(ω+p) t component can be obtained as a modulated signal. cos(ω+p) t is referred to as an upper side band and cos(ω−p) t is referred to as a lower side band. Here, ω>p.
The local oscillator 32 oscillates a reference signal cos ωt. The signal of cos ωt is inputted to the phase shifter 33. The phase shifter 33 outputs cos ωt and sin ωt whose phase is different from that of cos ωt by 90°. When Vin=cos(ω−p) t+cos ωt+cos(ω+p) t is inputted to the image rejection mixer 30A, the mixer 31a multiplies Vin by cos ωt. The LPF 34a removes a high band component of the multiplied signal and passes component with frequencies equal to or lower than a frequency p. As a result, cos pt is extracted with respect to the lower side band cos(ω−p) t and cos pt is extracted with respect to the upper side band cos(ω+p) t.
The mixer 31b multiplies Vin by sin ωt. The LPF 34b removes a high band component of the multiplied signal and passes components with frequencies equal to or lower than a frequency p. As a result, sin pt is extracted with respect to the lower side band cos(ω−p) t and −sin pt is extracted with respect to the upper side band cos(ω+p) t. The phase shifter 35A advances, by 90°, phases of outputs of the LPF 34b sin pt and −sin pt and converts them into −cos pt and +cos pt. When converted components of the lower side band are inputted, the adder 36 adds +cos pt to −cos pt and outputs only a DC component. Further, when converted components of the upper side band are inputted, the adder 36 adds +cos pt to +cos pt and outputs a signal of 2 cos pt. In this way, the lower side band is cancelled and only the component of the upper side band remains. Accordingly, the circuit shown in FIG. 2 serves as an image rejection mixer for canceling lower side band.
Structural elements of an image rejection mixer 30B shown in FIG. 3 are the same as those of FIG. 2 except that the phase shifter 35A shown in FIG. 2 is substituted by the phase shifter 35B and a phase is delayed by 90°. In this case, the image rejection mixer for canceling upper side band can be obtained. Referring to FIGS. 2 and 3, a circuit formed by the first mixer 31a, the second mixer 31b, the local oscillator 32 and the first phase shifter 33 is referred to as a quadrature mixer.
FIGS. 4A and 4B are spectral diagrams showing a channel where an analog TV signal is broadcast and an empty channel where an analog TV signal is not broadcast in a conventional VHF band. When digital terrestrial broadcasting formed by OFDM modulation or another modulation system is provided for an empty channel such as a taboo channel or the like, the conventional analog TV signal is positioned at an upper or a lower adjacent frequency band of the channel for digital broadcasting. FIG. 4A shows a case of receiving an upper side segment of a digital terrestrial sound broadcasting. FIG. 4B shows a case of receiving a lower side segment.
An above-described example of setting a frequency at the PLL 9 corresponds to FIG. 4B. An output of the divider 10 is a local oscillator frequency fLO by frequency conversion. An image rejection operation at this case suppresses an upper frequency component of the local oscillator frequency. For frequency conversion not by an image rejection mixer but by an ordinary mixer, frequency components positioned at a value of the upper 500 kHz of the local oscillator frequency (fLO+500 kHz) and at a value of the lower 500 kHz thereof (fLO−500 kHz) are converted into an intermediate frequency of 500 kHz. Since disturbance occurs in such frequency conversion, frequency components at unnecessary sides must be removed by a filer prior to the frequency conversion. An image rejection mixer has an advantage that such a filter prior to the frequency conversion is not required. Nevertheless, a degree of image suppression is deteriorated by quadrature errors of two local oscillation signals generated at the divider 10, quadrature errors of the phase shifters 13 and 14 and a difference of amplitude between an I axis intermediate frequency signal and a Q axis intermediate frequency signal. Thus, it is usually difficult to ensure a high degree of image suppression exceeding 30 dB.
Referring to FIG. 1, an intermediate frequency signal outputted from the adder 15 is supplied to the BPF 16. A center frequency of the BPF 16 is 500 kHz and a passband width thereof is equal to or larger than one segment. The BPF 16 removes interfering signal components such as other adjacent segments and an analog broadcasting signal of the adjacent channel, and selects a desired received segment.
An output of the BPF 16 is inputted to the base band amplifier (which hereinafter is referred to as BB amplifier) 17. The BB amplifier 17 is an amplifier having an AGC control function. The BB amplifier 17 amplifies an input signal to a set amplitude and supplies a resultant signal to the LPF 18. The LPF 18 removes unnecessary frequency components and supplies a result to the ADC 19. The ADC 19 converts an output of the LPF 18 into a digital signal while maintaining a center frequency at 500 kHz.
An output of the ADC 19 is supplied to the OFDM demodulator 20. The OFDM demodulator 20 performs demodulation processes such as complex Fourier transform, frequency deinterleave, time deinterleave and error correction in accordance with a modulation process at a time of sending ISDB-T. A demodulated result is outputted to the TS output terminal 21 as a transport stream (TS). A subsequent back end (not shown) reproduces a voice and an audio signal by decoding TS.
When TV broadcasting of ISDB-T for UHF band and sound broadcasting for VHF band are received, a receiver must receive a signal with a wide-band of 90 MHz to 770 MHz. In accordance with the structure of the conventional example, a local oscillator oscillates at a frequency twice or four times larger than a received frequency in order to ensure quadrature precision of the quadrature detection. Thus, an oscillation frequency of the local oscillator has a significant wide-band. When IC is performed such that a resonance circuit of the oscillator is built, a band must be divided into plural bands. At this time, since resonance circuits corresponding to the number of bands are required, a scale of the circuit is increased and thus the IC of the tuner portion is difficult. When a quadrature signal is generated not by a divider but by a phase shifter, it is difficult to ensure quadrature precision at a wide-band. Under such circumstances, it is required to realize a receiving apparatus which can receive a wide-band from VHF to UHF.
In order to solve such a drawback, a receiving apparatus of the second conventional example (U.S. Pat. No. 6,377,315) is provided. FIG. 5 shows a structure of the receiving apparatus. The receiving apparatus is configured so as to include an RF input terminal 41, an RF-AGC 42, a mixer 43, a first PLL 44, a first band pass filter (BPF) 45, a first mixer 46, a second mixer 47, a first poly phase filter (POLY PHASE) 48, a second PLL 49, a second poly phase filter 50, a second band pass filter (BPF) 51, an IF-AGC 52 and an output terminal 53.
An RF signal of VHF or UHF inputted from the RF input terminal 41 has a frequency band of 50 to 860 MHz, and is inputted to the RF-AGC amplifier 42 and amplified therein. The RF-AGC amplifier 42 is formed by a variable attenuation circuit and a low noise amplifier (LNA). An output of the RF-AGC amplifier 42 is inputted to the mixer 43. The PLL 44 is a local oscillator which oscillates at a frequency with a band of 1270 to 2080 MHz. The mixer 43 mixes an output of the RF-AGC amplifier 42 with an output of the PLL 44 so as to perform frequency conversion into a first intermediate frequency fIF1. The first intermediate frequency fIF1 is 1220 MHz.
The BPF 45 passes a signal component with the frequency fIF1 and removes adjacent signals. An output of the BPF 45 is inputted to the mixers 46 and 47. The PLL 49 oscillates a reference signal of 1176 MHz. The poly phase filter 48 converts the reference signal from the PLL 49 into two signals with a phase difference thereof being 90° and the signals are respectively applied to the mixers 46 and 47. The mixers 46 and 47 perform frequency conversion for a signal of frequency fIF1 by using a quadrature output of the poly phase filter 48.
The poly phase filter 50 composites output signals of the mixers 46 and 47 with a phase difference of 90° so as to remove an image band component. An image band refers to as a frequency component (fIF1−2×fIF2) wherein a frequency of the second intermediate frequency signal is fIF2. When an image rejection mixer is not used, conversion into a frequency fIF2 is performed as in a case of the frequency fIF1. If fIF2>>fIF1, an attenuation of the BPF 45 cannot be sufficiently ensured at an image band. Thus, an image rejection mixer is used in order to complement the attenuation. The BPF 51 passes the second intermediate frequency signal of the frequency fIF2 and removes adjacent signal components. Thereafter, the IF-AGC amplifier 52 adjusts amplitude to an optimum input level of the subsequent demodulation circuit (not shown) and outputs a result to the IF output terminal 53.
As described above, in accordance with the receiving apparatus of a second conventional example, image rejection is performed at a fixed frequency fIF1. Thus, reception in a wide-band such as VHF and UHF can be performed while ensuring a precision of image rejection.
Next, a case of receiving digital terrestrial sound broadcasting (ISDB-TSB) will be considered. In accordance with the digital terrestrial sound broadcasting, a service is independent for each segment. Thus, by receiving only one desired segment and performing a demodulation process for the segment, the sound broadcasting can be received. Above-described FIGS. 4A and 4B show a frequency spectrum for digital terrestrial sound broadcasting in a specific channel with 4 MHz of irregular bandwidth for analog TV broadcasting. The specific channel is the 7-th channel. In Tokyo and Osaka, practical test broadcasting for digital terrestrial sound broadcasting will take effect from the end of 2003.
In the channel, eight segments are connected together and broadcasted. FIG. 4A shows a case of receiving the top segment of eight segments. At this time, in order to suppress disturbance from picture carrier for upper adjacent analog TV broadcasting, a local oscillator (LO) for detection sets a frequency to this that is lower than a center frequency fRF of received segment by fIF. Then, the image rejection mixer operates so as to suppress signal components lower than fLO. If an LO is set so as to have a frequency which is higher than that of the received segment by the intermediate frequency fIF and signal components higher than fLO are suppressed by the image rejection mixer, a picture carrier of the upper adjacent NTSC signal must be suppressed. At this time, if the power of the interfering signal is relatively large, suppression cannot be performed thoroughly. Accordingly, when an intermediate frequency fIF is set to around 500 kHz and at least first and second segments from the upper end are received, the setting as shown in FIG. 4A is required.
On the other hand, FIG. 4B shows a case of receiving the bottom segment of eight segments. In order to suppress disturbance from a lower adjacent sound carrier for analog TV broadcasting, an LO for detection sets a frequency which is higher than a center frequency fRF of the received segment by fIF. Then, the image rejection mixer is operated so as to suppress signal components higher than fLO. Inversely, if the LO is set to a frequency which is lower than that of the received segment by the intermediate frequency fIF and signal components lower than fLO are suppressed by the image rejection mixer, the sound carrier of the lower adjacent NTSC signal must be suppressed. At this time, if the power of interfering signal is relatively large, suppression cannot be performed thoroughly. Accordingly, when the intermediate frequency fIF is set to around 500 kHz and at least the first and second segments from the lower end are received, setting as shown in FIG. 4B is required.
Next, a case of receiving all of the thirteen segments of ISDB-T for digital terrestrial TV broadcasting or an OFDM signal such as DVB-T or the like will be considered. FIG. 6A shows a frequency spectral diagram when a lower adjacent NTSC exists. A local oscillator frequency fLO of the image rejection mixer is set so as to satisfy the relationship fLO=fRF+fIF. In this case, the adjacent NTSC signal is outside an image band and thus disturbance of the image is reduced. Similarly, FIG. 6B shows a case where an upper adjacent NTSC exists. The local oscillator frequency fLO of the image rejection mixer is set so as to satisfy the relationship fLO=fRF−fIF. In this case, the adjacent NTSC signal is outside an image band and thus disturbance of the image is reduced.
As described above, in accordance with the image rejection mixer, in order to reduce disturbance of the image, an image band to be removed by the image rejection mixer must be switched between an upper band and a lower band depending on a position of the received segment in a case of digital terrestrial sound broadcasting. In a case of digital terrestrial TV broadcasting, an image band to be removed by the image rejection mixer must be switched between an upper band and a lower band depending on a position of the adjacent interfering signal. In order to realize such switching, the phase shifters 13 and 14 shown in FIG. 1 must be switched. Alternatively, an amount of phase shift of the phase shifters 13 and 14 must be switched. Nevertheless, there arise problems in that a gain difference between an I axis and a Q axis is easily generated by such operation and thus an image rejection performance cannot be sufficiently obtained. Further, in accordance with the structure of FIG. 5, little attenuation of the adjacent NTSC cannot be obtained at the BPF 45. Thus, a band for image rejection must be switched between an upper band and a lower band. There also arises a problem in that an image rejection performance is deteriorated by switching of the phase shifters or an amount of the phase shift.